Ultrasonic and megasonic wafer cleaning methods are known in the semiconductor industry, in particular for cleaning silicon wafers. The general principle is to bring the wafer into contact with a cleaning liquid, usually by submerging the wafer in a liquid-filled tank, and to apply acoustic energy to the cleaning liquid, by way of an acoustic transducer. Most known applications use acoustic waves in the ultrasonic (<200 kHz) or megasonic (up to or above 1 MHz) frequency range. In the presence of a gaseous substance dissolved in or added to the liquid, the acoustic energy causes cavitation, i.e. the creation of bubbles that oscillate or even collapse. The bubbles assist in the removal of particles from the wafer surface, due to the drag forces created by the bubble formation, the oscillation, or by drag forces created when bubbles become unstable and collapse. However, current techniques suffer from a number of problems.
At ultrasonic frequencies, resonant bubbles are large and collapse more heavily, leading to an increased risk of damaging the substrate and the structures present on it. Megasonic cleaning leads to smaller resonant bubbles and lower damage risk. However, as the structures present in integrated circuits are made smaller each new generation of technology, the damage risk remains. On the other hand, when the bubbles are smaller than resonant size, they do not sufficiently contribute to the removal of particles from the wafer surface. Presently known acoustic cleaning methods rely on the formation of standing waves in the cleaning liquid, due to reflection of acoustic waves off the surface to be cleaned. In a standing wave, bubbles are attracted towards nodes and antinodes. As a result, the bubble concentration can be very high locally. Those densely packed bubbles can influence each other (less control over bubble oscillation) and the bubble coalescence rate will be much higher (less control over bubble size).